1. Field of the Invention
This invention is concerned with analysis of alkaline copper electroplating baths as a means of providing control over the deposit properties.
2. Description of the Related Art
Copper is the preferred metallization for providing electrical interconnections on high-speed semiconductor chips due to its high electrical conductivity and good electromigration resistance compared to aluminum. However, copper is more difficult to pattern than aluminum so that it has been necessary to develop alternative methods for forming copper circuitry on semiconductor substrates. The leading technology for fabricating copper integrated circuit (IC) chips is the “Damascene” process (P. C. Andricacos, Electrochem. Soc. Interface, Spring 1999, p. 32; U.S. Pat. No. 4,789,648 to Chow et al.; U.S. Pat. No. 5,209,817 to Ahmad et al.), which depends on copper electroplating to provide complete filling of the fine features involved.
In the Damascene process, fine trenches and vias are patterned and etched into a dielectric layer of silicon dioxide on a silicon wafer using standard photoresist techniques. Copper is ultimately electrodeposited within the trenches and vias to form the circuitry but a barrier layer must be used to prevent migration of copper ions into the silicon dioxide dielectric material. The barrier layer is typically 100 to 300 Å of titanium nitride or tantalum nitride deposited by a physical vapor deposition (PVD) or chemical vapor deposition (CVD) method. A seed layer of copper, typically deposited by a PVD or CVD method, is usually needed to promote adhesion between the barrier layer material and the copper electrodeposited within the trenches and vias to form the circuitry. Copper electrodeposition is generally performed from an acid copper sulfate electroplating bath containing organic additives that provide complete filling of the Damascene features (trenches and vias).
Seed layers deposited by PVD or CVD methods tend to be thicker near the outsides of the Damascene features and thinner on the sidewalls and bottoms. Copper seed layers also undergo chemical dissolution in contact with the acidic plating bath on open circuit, before a cathodic voltage is applied to plate copper. The surface oxide formed on the seed layer in contact with air prior to copper plating dissolves rapidly in the acidic bath, resulting in a loss of seed layer copper. Consequently, a seed layer thickness of about 1000 Å is needed to provide adequate coverage of the barrier layer and good adhesion of the electrodeposited copper.
As the dimensions of Damascene features have been extended into the submicron region, the nonuniformity of PVD and CVD seed layers has become a significant problem. In particular, greater seed layer thickness near the top of a feature tends to pinch-off the feature entrance, further reducing the seed layer thickness on the sidewalls and bottom. The subsequent copper electrodeposition process is adversely affected by both the reduced seed layer thickness and restricted electrolyte flow within features so that complete filling of very fine features is not attained.
Prior art approaches to addressing this problem have involved developing an alkaline bath for electroplating uniform copper layers directly on the barrier layer, or on an ultra-thin copper seed layer (used to promote adhesion to the barrier layer). The alkaline bath provides a conformal copper coating of uniform thickness so that a thicker copper seed layer can be deposited without pinching-off the trenches. In addition, copper does not readily dissolve in the alkaline solution and surface copper oxides tend to be reduced back to adherent metal at the beginning of the electrodeposition process so that seed layer loss in the alkaline bath is minimal. The alkaline bath typically operates at a relatively low current density and is only used to provide a conformal copper seed layer of sufficient thickness to survive in an acid copper bath, which is used to rapidly fill the Damascene features.
U.S. Pat. No. 5,151,168 to Gilton et al. describes an alkaline copper electroplating bath for depositing conformal copper coatings directly on barrier layer materials, such as titanium nitride, titanium-tungsten or nitrided titanium-tungsten. In a preferred embodiment, the bath comprises 0.035 M cupric sulfate and 0.07 M sodium ethylenediaminetetraacetate (NaEDTA) complexing agent (pH 13.5), and is operated at 1 mA/cm2 at 25° C.
U.S. Pat. Nos. 6,197,181, 6,277,263 and 6,919,013 to Chen describe an alkaline copper electroplating bath for conformally enhancing an ultra-thin copper seed layer (preferably 200 Å thick and deposited by a PVD method) for subsequent Damascene plating from an acid copper electroplating bath. The disclosed alkaline plating bath comprises cupric sulfate (0.03 to 0.25 M), a complexing agent (molar ratio of 1 to 4 relative to copper ion), and potassium hydroxide, ammonium hydroxide, tetraethylammonium hydroxide or sodium hydroxide (pH at least 9.0). The bath preferably also includes 0.01 to 0.5 M boric acid to aid in maintaining pH and/or to improve deposit quality. Salts with electrochemically unreactive cations, 0.01 to 0.5 M ammonium sulfate, for example, may be added to increase the electrolyte conductivity so as to reduce the sheet resistance. In this case, it may be advantageous to add an agent, such as ethylene glycol, to enhance the conformality of the deposited copper layer.
Suitable complexing agents disclosed by Chen ('181, '263 and '013) consist of ethylene diamine (EDA), ethylene diamine tetraacetic acid (EDTA), a polycarboxylic acid, such as citric acid, and salts thereof. These complexing agents may be used in combination with each other or with other complexing agents. Preferred pH values are 9.5 for baths containing citric acid or EDA as the complexing agent, and 12.5 for baths employing the EDTA complexing agent. The bath may be operated at a temperature in the range 20° to 35° C. (preferably 25° C.), and a current density in the range 1 to 5 mA/cm2 (preferably 1 to 2 mA/cm2) may be used. The copper layer plated on the seed layer from the alkaline copper bath preferably has a thickness in the 400 to 800 Å range.
One preferred plating bath according to Chen ('181, '263 and '013) has a pH of 9.5 and comprises 0.1 M cupric sulfate (CuSO4), 0.2 M citric acid and 0.05 M boric acid (H3BO3). Another preferred plating bath according to Chen ('181, '263 and '013) has a pH of 9.5 and comprises 0.25 M cupric sulfate, 0.5 M EDA, 0.2 M boric acid and 0.3 M ammonium sulfate. Yet another preferred plating bath according to Chen ('181, '263 and '013) has a pH of 12.5 and comprises 0.1 M cupric sulfate, 0.2 M EDTA and 0.05 M boric acid.
U.S. Pat. No. 7,135,404 to Baskaran et al. describes an alkaline copper electroplating bath for Damascene seed layer applications having the same constituents as that of Chen ('181, '263 and '013) but with extended concentration limits. According to Baskaran '404, suitable alkaline copper baths for depositing conformal copper coatings with good adhesion comprise cupric sulfate (0.004 to 1.0 M), complexing agent (molar ratio of 1 to 4 relative to copper ion), boric acid (0.001 to 0.5 M) and tetramethylammonium hydroxide (pH 9.5 to 12.5). A bath comprising 10 g/L cupric sulfate, an ethylene diamine (EDA) complexing agent (molar ratio of 2 relative to copper ion), 3.1 g/L boric acid and tetraethylammonium hydroxide (pH 9.5) is reportedly suitable for depositing seed layers directly on barrier layer materials. Baskaran '404 also discloses that the source of copper ions in the plating bath may be copper sulfate, copper gluconate, sodium copper cyanide, copper sulfamate, copper chloride, copper citrate, copper fluoroborate or copper pyrophosphate. Baskaran '404 further discloses that the alkaline copper electroplating bath may also include an alloying metal selected from the group consisting of chromium, nickel, cobalt, zinc, aluminum, boron, magnesium and cerium. A suitable bath for plating a copper-chromium alloy seed layer directly on barrier layer materials reportedly comprises CrSO4 (10-40 g/L), CuSO4 (5-20 g/L), (NH4)2SO4 (20-40 g/L), NH4OH (50-100 mL/L), and EDA or EDTA (0.1-1.0 mL/L).
Such prior art alkaline plating baths are intended to provide the conformal copper seed layers needed for complete filling of very fine Damascene trenches and vias with void-free copper having good adhesion to the barrier layer. In order to obtain consistently good results, the concentrations of the key constituents in the plating bath, copper ions and the complexing agent, must be closely controlled. It is likely that a titration method based on a specific complexing agent titrant could be developed to measure the copper ion concentration, and that a titration method based on a metal ion titrant could be developed to measure the complexing agent concentration. However, such an approach is undesirable since Damascene plating processes, including the plating bath analyses, must be highly automated, and use of multiple reagents renders automation more difficult. In addition, it is desirable to minimize the waste stream from the Damascene process for environmental reasons. Consequently, a simple method is needed for measuring the copper ion and complexing agent concentrations in alkaline copper electroplating baths.